Create structures people are proud to improve.

People improve things they respect and do the minimum on things they find shoddy. A structure that shows care invites care, and one that is visibly held together with workarounds teaches everyone that quality is not expected here. The standard you set is contagious in both directions.

If you want better contributions, raise the quality of what people are contributing to. Fix the embarrassing parts, tidy the entry points, make it something a good person would want their name on. The behaviour usually follows the environment.